The present invention relates to a process for the preparation of a bundle of discontinuous filaments which is an intermediate product for use in preparing spun yarns from a bundle of continuous filaments, such as a tow or multifilament.
As the known process for the preparation of spun yarns, there can be mentioned a process in which staple fibers are passed through the carding step, the gilling or drawing step, the roving step, and the spinning step. This process, however, involves the following problems because it comprises the carding step:
(1) The productivity is low.
(2) Shrinkability cannot be given to a spun yarn at the spinning step.
(3) At the start of the fiber-preparing step, it is necessary to cut the fibers into stable fibers having a length suitable for the object of spinning.
(4) At the carding step, neps (entanglements of single fibers) and hooks (bends of the top ends of single fibers) are formed, and the degree of parallelization or parallelism in single fibers is low. Hence, means for prolonging the gilling step or other special means become necessary.
As the process comprising converting a bundle of continuous filaments, such as a tow or multifilament, into a bundle of discontinuous filaments and forming a spun yarn from the bundle of discontinuous filaments, there is known a process in which a bundle of discontinuous filaments is formed at a temperature close to room temperature according to the Perlohrke system or Turbo system.
According to the Perlohrke system, a bundle of continuous filaments is drawn by rollers to cut the respective single filaments, the intention being to obtain a bundle of discontinuous filaments having a high degree of parallelization at a high speed. As can be seen from the strength-elongation curve (determined at a temperature of 20.degree. C. and a relative humidity of 65%) of acrylic-type synthetic fibers (marketed under the tradename of Cashmilon.RTM.), shown in FIG. 10, at the cutting step of this method, filaments are passed through a region of elastic deformation in which the filaments are stretched up to about 5% and then are passed through a region of plastic deformation in which the filaments are stretched more than 5%, i.e., the filaments are stretched to the point of elongation at breakage to effect cutting of the filaments. Accordingly, the following problems arise when the Perlohrk system is adopted:
(1) Under ordinary spinning conditions, a large residual strain is given to filaments by the cutting operation, and, therefore, the production of spun yarns having a low shrinkage degree is limited.
(2) Since the strength and elongation, especially the loop elongation and loop strength, are drastically reduced by the cutting operation, breakage of the filaments or flying of the filaments frequently occurs in the process for preparing spun yarns.
(3) When filaments having a high elongation are drawn and cut, since preliminary drawing is first carried out and draw-cutting is then effected according to the Perlohrke system, the defect mentioned in the preceding paragraph (2) is augmented.
(4) The top ends of cut filaments become frizzled, resulting in a reduction of the quality in spun yarns, especially in direct spun yarns.
According to the Turbo system, a bundle of continuous filaments is cut by applying a shearing force while the bundle is being drawn. In this method, it is not necessary to stretch filaments to the point of elongation at breakage, but the above-mentioned defects (1) through (3) are not substantially eliminated, and the staple diagram of the cut filaments is degraded, that is, the amount of excessively long fibers and short fibers is increased.